New measurements of rotation and differential rotation in A-F stars: are there two populations of differentially rotating stars? ^{⋆,⋆⋆,⋆⋆⋆}

¹ Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
e-mail: ammler@tls-tautenburg.de
² Institut für Astrophysik, Georg-August-Universität Göttingen, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
e-mail: areiners@astro.physik.uni-goettingen.de
³ Max Planck Institute for Solar System Research, Max-Planck-Straße 2, 37191 Katlenburg-Lindau, Germany

Received: 22 December 2011
Accepted: 25 March 2012

Abstract

Context. The Sun displays differential rotation that is intimately connected to the solar dynamo and hence related to solar activity, the solar cycle, and the solar wind. Considering the detectability and habitability of planets around other stars it is important to understand the role of differential rotation in other stars.

Aims. We present projected rotational velocities and new measurements of the rotational profile of some 180 nearby stars with spectral types A-F. The results are consolidated by a homogeneous compilation of basic stellar data from photometry and the identification of multiple stellar systems. New and previous measurements of rotation by line profile analysis are compiled and made available.

Methods. The overall broadening profile is derived analysing spectral line shape from hundreds of spectral lines by the method of least-squares deconvolution, reducing spectral noise to a minimum. The effect of differential rotation on the broadening profile is best measured in inverse wavelength space by the first two zeros of its Fourier transform.

Results. Projected rotational velocity vsini is measured for more than 110 of the sample stars. Rigid and differential rotation can be distinguished in 56 cases where vsini > 12   km   s^-1. We detect differential rotation rates of δΩ/Ω = 5% and more. Ten stars with significant differential rotation rates are identified. The line shapes of 46 stars are consistent with rigid rotation, even though differential rotation at very low rates might still be possible in these cases. The strongest amount of relative differential rotation (54%) detected by line profile analysis is found among F stars.

Conclusions. As of now, 33 differential rotators detected by line profile analysis have been confirmed. The frequency of differential rotators decreases towards high effective temperature and rapid rotation. There is evidence for two populations of differential rotators, one of rapidly rotating A stars at the granulation boundary with strong horizontal shear and one of mid- to late-F type stars with moderate rates of rotation and less shear. The gap in between can only partly be explained by an upper bound found for the horizontal shear of F stars. Apparently, the physical conditions change at early-F spectral types. The range of horizontal shear observed for mid-type F stars is reproduced by theoretical calculations while there seems to be a discrepancy in period dependence for late-F stars.